containerd support with nerdctl

[arothste-blk]

are you open to PR that adds support for containerd runtimes with nerdctl?

[stepchowfun]

I see that nerdctl events is not compatible with docker events, so we would need to introduce some way to parse the events from nerdctl. I would support this if we can do it in a way that will eventually allow us to close similar feature requests, such as #226 and #134.

I'm imagining a command-line flag that allows the user to select which Docker-like tool they want to use.

[stepchowfun]

(We would also need an integration test for it.)